1. Field of the Invention
This invention relates generally to the field of coded squelch signalling for two-way radio communications and more particularly to an improved signalling arrangement providing enhanced privacy in a multiple user two-way radio channel.
2. Background of the Invention
Various forms of coded squelch have been utilized to enable large numbers of users to occupy a single radio frequency channel while maintaining privacy of conversation within selected groups of users having the same code. Two such techniques of squelch coding are described in Motorola Technical Training Manual Number TT500 entitled Portable Products - "Digital Private-Line Coded Squelch", the contents of which is hereby incorporated by reference. In these two squelch coding techniques, a tone or a digital code word is continuously transmitted along with voice information in order to enable the receiving radios audio circuitry. Only those radios having the proper tone code or digital code word will respond to such appropriately encoded transmitted signals.
In another type of coded squelch system, individual users, repeaters or groups of users are assigned individual identification codes which may be either a digital word or a sequence of tones having a particular set of frequencies. This code is transmitted at the first of a communication to establish contacts between two users. After the initial contact conversation is carried out using uncoded squelch or carrier squelch.
Each of these systems have the inherent disadvantage of allowing privacy only between particular groups having different codes. All users having the same code have access to the conversation but cannot communicate outside their assigned group. In the latter signalling arrangement the majority of the conversation must be carried out in the carrier squelch mode. In each system, selective calling any single user of a large group is not practical. The present invention overcomes these difficiences.